3.40/22.71 Summary of 11/29/2012 Sergio Castellanos Mechanical Engineering Department Massachusetts Institute of Technology, Cambridge, MA (USA)

Fe-C Phase Diagram Not to scale 1130 ºC 723 ºC γ (Austenite) δ (Ferrite) α+γ α+Cementite α (ferrite) Cementite (Fe 3 C) γ+Cementite γ+Liquid BCC % 912 ºC 1394 ºC 2.11% FCC Carbon (% by Weight) Liquid Solid

Martensite Transformation Austenite Martensite Large Cooling Rate FCC BCT * Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid. ISBN © 2006 x x Scalar Vectorial (BCC)

 Displacive: C diffusion is restricted tr ε = 0 ~  Bain Transformation: Constant Volume Martensite Transformation Twinning Strain (Simple Shear) [1] Lieberman, Acta Metallurgia 6 (1958) [2] H. Ledbetter, M.L. Dunn, Materials Science and Engineering A (1999)

Twinning Energy: Least if formed into disk ΔGΔG But all at once? Good thing: w proportions cancel strain fields Wechsler-Lieberman-Read (WLR) Theory a Still incur in expense: Interface Energy Coherent TB A.L. Roitburd and G.V. Kurdjumov, Mater. Sci. & Eng, 39 (1979), 141.

Elastic Energy vs. Interfacial Energy ….. Steels: 10’s nm bi-layer

Importance of Martensitic Transformation Utilization: t Hardness (Rockwell) Tempering: 200 ºC 600 ºC Image credit: Difussion process Cementite in Ferrite matrix

Shape-Memory Alloys   T Cooling Detwinning Heating/Recovery Image Credits: [1] Texas A&M – Active Materials Laboratory [2]

Thank you Image credit:

 Martensite-Austenite interface structure Appendix